ABSTRACT

Detailed diagenetic studies of the late Cambrian Alum Shale in southern Sweden were undertaken across an interval that includes the peak Steptoean Positive Carbon Isotope Excursion (SPICE) event to evaluate the pyrite mineralization history in the formation. Samples were collected from the Andrarum-3 core (Scania, Sweden); here the Alum was deposited in the distal, siliciclastic mudstone-rich end of a shelf system. Abundant cryptobioturbation is observed in the Alum, which points to oxic–dysoxic conditions prevailing during deposition. Petrographic examination of polished thin sections (n = 65) reveals the presence of numerous texturally distinct types of pyrite, including matrix framboids, two different types of framboid concretions (those with rims of iron-dolomite and those lacking rims), disseminated euhedral pyrite crystals, concretions of euhedral pyrite crystals, overgrowths of pyrite on these different pyrite generations, anhedral pyrite intergrown with bedding parallel mineralized fractures (i.e., “beef”), and massive vertical/subvertical accumulations of pyrite.

Paragenetic relationships outline the relative timing of formation of the texturally distinct pyrite. Framboids and framboid concretions formed prior to precipitation of any euhedral pyrite crystals, and these pyrite generations precipitated prior to the pyrite overgrowths on them. As Alum Shale sediments are all distorted by these texturally different pyrite generations, they are likely to have formed early in the postdepositional history of the formation. In contrast, pyrite associated with “beef” is likely temporally related to the onset of hydrocarbon generation, which in this part of Sweden is thought to have been many tens of millions of years after deposition. Because vertical/subvertical massive pyrite features distort “beef,” they clearly postdate it. Of all these pyrite textures, only framboid concretions appear to be restricted to the SPICE interval.

The texturally distinct nature of the pyrite generations, along with evidence of their formation at different times in the postdepositional history of the Alum Shale, is the key outcome of this petrographic study. Because the petrographic data presented herein point to a postdeposition origin for all generations of pyrite, diagenetic processes—not those processes associated with deposition—were responsible for the complex pyritization history observed in the Alum, in the Andrarum-3 core.

INTRODUCTION

Pyrite and other sulfides are common authigenic minerals in organic-rich mudstones and can form when reducing conditions, along with adequate metals (e.g., iron, zinc) and sulfide, prevail. Reducing conditions may exist at the time of deposition if euxinic conditions exist in the water column above the sediment–water interface (Gill et al., 2011), although after deposition reducing conditions may be pronounced and more persistent in organic-rich sediments (e.g., Berner, 1970, 1985; Sweeney and Kaplan, 1973; Claypool et al., 1980; Raiswell et al., 2011; Taylor and Macquaker, 2011; Macquaker et al., 2014; Fishman et al., 2015). Determining the timing of pyrite authigenesis bears directly on understanding syndepositional and postdepositional processes related to the mudstones. This is the case for the marine Middle Cambrian (Guzhangian) through Lower Ordovician (Tremadocian) Alum Shale in southern Sweden.

The Alum Shale has been the subject of numerous studies, largely because of its significance as a source rock for hydrocarbons (e.g., Horsfield et al., 1992; Buchardt et al., 1997; Pedersen et al., 2007; Jarvie, 2012; Pool et al., 2012; Gautier et al., 2013) as well as a sink for metals (e.g., Holland, 1979; Andersson et al., 1985; Dahl et al., 1988; Leventhal, 1993). Within the Alum, in the Andrarum-3 core (Figure 1), there is a well-defined carbon isotopic excursion (see Ahlberg et al., 2009) that records the Steptoean Positive Carbon Isotope Excursion (SPICE), one of the presumed Paleozoic oceanic anoxic events (Saltzman et al., 2000; Gill et al., 2011). A recent investigation found that there was a sulfur isotopic excursion coincident with the SPICE event in the Alum in the Andrarum-3 core, Sweden (Gill et al., 2011), and the sulfur isotopic excursion was modeled to be from syndepositional pyrite that precipitated from a euxinic water column (Gill et al., 2011). Contrasting the euxinic water column model proposed by Gill et al. (2011) are the conclusions drawn from a detailed investigation of the same core (Egenhoff et al., 2015) that, based on sedimentological and ichnological features, indicate oxic to dysoxic conditions prevailed at the sediment–water interface during Alum deposition. These notably different conclusions describing conditions at the time the Alum was deposited prompted a comprehensive petrographic investigation of the same Alum samples used by Egenhoff et al. (2015) to evaluate the nature and textural features of pyrite in the Alum. The underlying assumption of this study was that textural features revealed by careful analysis of minerals in the Alum, particularly pyrite and other temporally coeval minerals, could be used to better understand the timing of formation—syndepositional or postdepositional—of pyrite that occurs in the Alum and thereby help determine whether pyrite mineralization occurred during deposition or diagenesis. If the textural evidence revealed the pyrite was syndepositional, then euxinic bottom waters may have led to the pyrite, whereas textural evidence could indeed point to a diagenetic origin to the pyrite, which would call into question the occurrence of euxinic conditions during Alum deposition.

The Andrarum-3 core from Scania, Sweden (Figure 1), which was the source of the material used in this study (Figure 2), contains about 29 m (95 ft) of the Alum Shale (Middle to Upper Cambrian) that has been subdivided into seven biozones based on trilobites and agnostids (Ahlberg et al., 2009). In their sedimentological study, Egenhoff et al. (2015) found the Alum to contain three mudstone and one carbonate facies arranged in four third-order cycles on top of an overall deepening depositional setting. The mudstone facies include facies 1 (massive clay clast-rich mudstone), facies 2 (laminated macrofossil debris-rich mudstone), and facies 3 (millimeter-laminated mudstone). The presence of fecal strings (elongate accumulations of fecal pellets that were originally multidirectional) throughout all mudstone facies as well as Planolites burrows (Figure 3) point to Alum deposition in a largely oxic to dysoxic environment, with little evidence of persistent anoxic conditions (Egenhoff et al., 2015).

Measured section of the Andrarum-3 drill core (modified from Egenhoff et al., 2015) showing the stratigraphic position, within the Alum Shale, of samples used in this study (sample designations are all AN-10- with the addition of the sample depth, in meters). The SPICE interval, as shown, and the sulfur isotopic data are from Gill et al. (2011). The trilobite zones are from Ahlberg et al. (2009).

Measured section of the Andrarum-3 drill core (modified from Egenhoff et al., 2015) showing the stratigraphic position, within the Alum Shale, of samples used in this study (sample designations are all AN-10- with the addition of the sample depth, in meters). The SPICE interval, as shown, and the sulfur isotopic data are from Gill et al. (2011). The trilobite zones are from Ahlberg et al. (2009).

Photomicrograph of a Planolites burrow (encircled by white dashed line) in the Alum Shale. Note that inside the burrow, there are largely detrital mineral grains (largely clay minerals with some silt-zie quartz and other detritus) and a paucity of organic material and pyrite whereas outside of the burrow organic material and pyrite are abundant and appear as black material in matrix. The burrow is parallel to bedding. Photomicrograph taken in plane, transmitted light, and the sample is from a depth of 12.95 m (42.49 ft).

Photomicrograph of a Planolites burrow (encircled by white dashed line) in the Alum Shale. Note that inside the burrow, there are largely detrital mineral grains (largely clay minerals with some silt-zie quartz and other detritus) and a paucity of organic material and pyrite whereas outside of the burrow organic material and pyrite are abundant and appear as black material in matrix. The burrow is parallel to bedding. Photomicrograph taken in plane, transmitted light, and the sample is from a depth of 12.95 m (42.49 ft).

METHODS

Petrographic examination (transmitted and reflected light microscopy) was undertaken on ultrathin (~20 μm thick) polished thin sections (n = 65, see Figure 2 for sampling intervals) from the Andrarum-3 core. All sections were cut from chips oriented perpendicular to bedding. Reflected light microscopy, using vertical illumination, in air, was used to determine the mineralogy of opaque phases. The absolute size of different types of pyrite were measured (Table 1), and although these dimensions reflect the size of features in the plane of the thin section, sufficient measurements were made to ensure that the range in size reasonably reflects the true dimensions of the mineral grains in the samples. A variably colored tarnish that was possible to remove with a gentle cleaning developed on the exposed surfaces, which indicates it was only a surface feature. The tarnish was preserved for viewing because it aided in identifying textural features and paragenetic relations.

Table 1.

Petrologic data on different pyrite textures, Alum Shale samples. *, range based on the diameters of at least 10 randomly selected samples; nd, No data; No, not observed; Present, feature observed but not quantified or measured.

The polished thin sections were eventually coated with carbon for use in examination with a scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDS). Both secondary and backscattered electron images were acquired. The backscatter coefficient (η) is related to the average atomic number of the mineral, so that the brighter the area, the higher the average atomic number (see Goldstein et al., 1981).

Semi-quantitative mineralogy was determined by x-ray diffraction (XRD) data collected on powdered bulk samples (n = 10) matched to the thin sections (Table 1). Whole rock mineralogy was performed to measure the abundance of the minerals present, including clay minerals. XRD data were collected on randomly oriented powdered samples using Cu Kα radiation; scans were from 3 to 80° 2θ, using scanning parameters of 0.02 steps and a 2-s count time per step. Mineralogy was calculated using Material Data, Inc. Whole Pattern Fit software, which simultaneously calculates a whole pattern fit and a Rietveld refinement of the minerals.

RESULTS

Numerous authigenic minerals were identified and, where possible, their paragenetic relationships were evaluated as part of the petrographic examination of Alum Shale samples. Below, petrographic data are presented that document the occurrence and distribution of texturally distinct generations of pyrite. Also listed are other rock characteristics that are germane to corroborating the temporal relationships of texturally different pyrite.

Pyrite Mineralization

Pyrite mineralization is present throughout the Alum Shale interval studied, although pyrite textures are numerous and variable in terms of stratigraphic distribution. Quantitative XRD data indicate that pyrite contents are in excess of 4.0 wt. % (Table 1) and reach as much as 13.7 wt. %. In general, for the limited number of samples analyzed by XRD, pyrite is present in amounts less than 5.3 wt. % either above or below the SPICE interval, whereas pyrite amounts range from 5.7 to 13.7 wt. % within the SPICE interval (Table 1). Because the XRD values represent bulk amounts of pyrite in the samples, it is necessary to evaluate the different textures by which pyrite is present in the rock to provide the context for the amount of pyrite present in the samples as determined by XRD.

Pyrite Crystallites

Scattered crystallites of pyrite occur in the matrix of all samples (Table 1), irrespective of depositional facies. The scattered pyrite crystallites were observed by both reflected light petrography and SEM examination (Figure 4A). They occur as individual small (≪1 μm across) euhedral to subhedral grains or accumulations of several grains (up to ~15 crystallites) that are adjacent to the surfaces of clay minerals, between clay platelets, and other presumed detrital grains (Figure 4A).

Photographs of authigenic sulfide textures from the samples of the Alum Shale. (A) SEM image of pyrite crystallites (arrows) within organic material (dark η) that surrounds detrital clay minerals. Note that the individual pyrite crystallites are ≪1 μm across. Also note the present of framboids (B) of varying sizes. Photograph taken in SEM-backscatter mode from sample depth of 7.00 m (22.97 ft). (B) Photomicrograph of a rimless framboid concretion (arrows) that is elongate parallel (horizontal) to bedding. Note that concretion is composed of many individual framboids. Photomicrograph taken in vertically reflected light in air; sample is from depth of 12.00 m (39.37 ft). (C) Photograph of a rimmed framboid concretion (rim outlined in white, dashed line). The interior of the concretion is composed of numerous individual framboids, whereas the rim is largely Fe-dolomite (determined from SEM–EDS analyses). Note that bedding is distorted above and below concretion. Photograph taken in SEM-backscatter mode from a sample depth of 12.50 m (41.01 ft). (D) Photomicrograph of euhedral pyrite grain (center of photo), with numerous smaller subhedral pyrite crystals (white arrows) and anhedral pyrite crystals (yellow arrows) in the surrounding matrix. Note how bedding is distorted around the large euhedral pyrite crystal. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 12.50 m (41.01 ft). (E) Photomicrograph of a concretion composed of euhedral pyrite crystals. Note that the pyrite demonstrates various colors resulting from tarnish; it remains unclear why the crystals display a differing tarnish. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 2.77 m (9.09 ft). (F) Photomicrograph of euhedral pyrite crystals dispersed in the matrix of the Alum Shale. Note the presence of a framboid at the core of many of the pyrite crystals (arrows) that now display a euhedral texture. The tarnish of the rims of authigenic pyrite on framboids assists in identification of the framboids within the euhedral pyrite. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 21.00 m (68.90 ft).

Photographs of authigenic sulfide textures from the samples of the Alum Shale. (A) SEM image of pyrite crystallites (arrows) within organic material (dark η) that surrounds detrital clay minerals. Note that the individual pyrite crystallites are ≪1 μm across. Also note the present of framboids (B) of varying sizes. Photograph taken in SEM-backscatter mode from sample depth of 7.00 m (22.97 ft). (B) Photomicrograph of a rimless framboid concretion (arrows) that is elongate parallel (horizontal) to bedding. Note that concretion is composed of many individual framboids. Photomicrograph taken in vertically reflected light in air; sample is from depth of 12.00 m (39.37 ft). (C) Photograph of a rimmed framboid concretion (rim outlined in white, dashed line). The interior of the concretion is composed of numerous individual framboids, whereas the rim is largely Fe-dolomite (determined from SEM–EDS analyses). Note that bedding is distorted above and below concretion. Photograph taken in SEM-backscatter mode from a sample depth of 12.50 m (41.01 ft). (D) Photomicrograph of euhedral pyrite grain (center of photo), with numerous smaller subhedral pyrite crystals (white arrows) and anhedral pyrite crystals (yellow arrows) in the surrounding matrix. Note how bedding is distorted around the large euhedral pyrite crystal. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 12.50 m (41.01 ft). (E) Photomicrograph of a concretion composed of euhedral pyrite crystals. Note that the pyrite demonstrates various colors resulting from tarnish; it remains unclear why the crystals display a differing tarnish. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 2.77 m (9.09 ft). (F) Photomicrograph of euhedral pyrite crystals dispersed in the matrix of the Alum Shale. Note the presence of a framboid at the core of many of the pyrite crystals (arrows) that now display a euhedral texture. The tarnish of the rims of authigenic pyrite on framboids assists in identification of the framboids within the euhedral pyrite. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 21.00 m (68.90 ft).

Framboidal Pyrite

Framboidal pyrite occurs in several different textural features in the Alum Shale. First, singular framboids occur disseminated throughout the matrix (Figure 4A) and range in size from 3 to 35 μm across (Table 1). For the matrix framboids, individual pyrite crystals can be separated by what appears to be void space or can be virtually in contact with each other (Figure 4A). Regardless of the intercrystalline space between individual pyrite crystals, they are all herein referred to as matrix framboids. The singular framboids occur throughout the sampled interval of core, although there is variability in diameter as a function of stratigraphic position. Individual matrix framboids range from ~3 to 10 μm in diameter for samples in the upper part of the cored interval (depth: 3.30–7.00 m [10.8–23 ft]). In contrast, matrix framboids range up to 25 μm in diameter in the depth interval of 7.50–12.50 m (24.6–41 ft). Framboids in the matrix in the depth interval of 13.80–18.30 m (45.3–60 ft) range in diameter from 2 to 6 μm, but below ~19.00 m (~62.3 ft), they are as large as 35 μm.

Framboids also occur in the Alum Shale as distinct accumulations or aggregates. Within the plane of the thin sections, these features are elongate parallel/subparallel to bedding; in hand sample, these features are oval in the third dimension. These accumulations of framboids have a horizontal dimension of generally less than 1000 μm, although they can be as much as 1700 μm across (Table 1) and typically distort bedding above and below them. These features characteristically lack an obvious external rim of authigenic minerals nor is there any indication that the framboids are enveloped within an organic rim (Figure 4B), such as would be the case if the framboids formed within an algal cyst or fossil fragment (cf. Schieber, 2011). Given their shape and size, these masses are herein termed rimless framboid concretions. Individual framboids within them range from 5 to 15 μm in diameter, and there may be up to several hundred framboids within each rimless framboid concretion. Rimless framboid concretions are not restricted to a single facies and are common from depths of 3.70–17.00 m (12.1–55.8 ft); they are much less common at greater depths (Table 1).

A second type of framboid concretion is present in the Alum Shale, and they are texturally distinct from rimless framboid concretions. Characteristically, this second type of framboid concretion has a rim of authigenic minerals (Figure 4C), which is used to distinguish them as rimmed framboid concretions. The authigenic mineralogy of the rims is principally ferroan dolomite (Fe-dolomite), as determined by SEM–EDS analysis, although pyrite, barite, melanterite, and other minerals also occur in them. Individual framboids range from 5 to 15 μm in diameter, and there may be up to several hundred framboids within each concretion. These concretions are elongate parallel/subparallel to bedding and commonly have a horizontal dimension greater than 1000 μm; locally they are as much as 10,000 μm in length (Table 1). Rimmed framboid concretions distort overlying and underlying bedding (Figure 4C). These concretions are facies independent but are stratigraphically restricted to the depth range of 5–17 m (16.4–55.8 ft) (Table 1).

Euhedral-Subhedral-Anhedral Pyrite

Pyrite, present as euhedral, subhedral, and anhedral crystals, is common in the Alum Shale and occurs in several texturally distinctive forms. First, individual euhedral pyrite crystals, up to 425 μm in diameter, occur disseminated in the matrix (Figure 4D) and were observed in all facies, although they were not present in all samples (Table 1). Where large enough (>100 μm), bedding is distorted around them (Figure 4D). A second textural feature is as accumulations of individual euhedral/subhedral pyrite crystals in oval masses (Figure 4E), herein referred to as pyrite concretions, which range up to 1000 μm in length (Table 1). These concretions are parallel/subparallel to bedding and distort the bedding around them. A third textural feature of euhedral/subhedral pyrite crystals is as a local enrichment in individual laminae. Finally, euhedral/subhedral pyrite is noted with framboids or relict framboid textures present in their cores (Figure 4F). Anhedral pyrite occurs largely with euhedral/subhedral pyrite.

Pyrite Overgrowths

Overgrowths of pyrite, accentuated by tarnishing, have been observed on most of the different textures of pyrite previously described. Framboids, whether in the matrix or in either type of framboid concretion, demonstrate overgrowths (Figure 5A), ranging from less than 1 to 3 μm in thickness. Both the individual pyrite crystallites in the framboids and the entire framboids are overgrown with pyrite (Figure 5A). Based on differential coloration of tarnish, multiple successive overgrowths seem apparent on some framboids (Figure 5A), and locally, the overgrowths appear to merge such that they cement adjacent framboids (Figure 5B). Pyrite overgrowths on euhedral pyrite crystals are also common, but the differential tarnish of the overgrowth relative to the pyrite core is required to distinguish them as separate features; without the tarnish, a grain such as that seen in Figure 5C would appear to be anhedral. Multiple, texturally distinct overgrowths have also been observed on euhedral pyrite crystals (Figure 5D).

Photomicrographs of different generations of authigenic pyrite in the Alum Shale. (A) Photomicrograph of overgrowths (OV) of pyrite on matrix framboids (B) as well as euhedral pyrite crystals. The variability of the tarnish of the pyrite assists in observing the various generations of pyrite observed associated with the framboids, including the pyrite crystals that comprise the framboid (light blue), pyrite that cements the intraframboidal region (orange-tan), and overgrowths on the entire framboid (both light blue and orange-tan). Also note the variably tarnished overgrowths on euhedral pyrite crystals with no obvious framboidal cores. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 26.28 m (86.22 ft). (B) Photomicrograph of pyrite cement (PCe) that occupies areas between framboids in rimmed framboid concretion. Clearly the cement postdates formation of the framboids within the concretion. Dark areas between the framboids and pyrite cement are Fe-dolomite cement. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 10.32 m (33.86 ft). (C) Photomicrograph of an anhedral pyrite overgrowth (OV) on the surface of a euhedral matrix pyrite crystal (P). The variation in tarnish color between the euhedral crystal (bluish-tan) and the anhedral overgrowth (orange-tan) assists in deciphering the two generations. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 3.70 m (12.14 ft). (D) Photomicrograph of multiple generations of overgrowths on small euhedral pyrite crystals in the matrix. The different generations of overgrowths are apparent based on the variability in tarnish color displayed. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 27.51 m (90.26 ft).

Photomicrographs of different generations of authigenic pyrite in the Alum Shale. (A) Photomicrograph of overgrowths (OV) of pyrite on matrix framboids (B) as well as euhedral pyrite crystals. The variability of the tarnish of the pyrite assists in observing the various generations of pyrite observed associated with the framboids, including the pyrite crystals that comprise the framboid (light blue), pyrite that cements the intraframboidal region (orange-tan), and overgrowths on the entire framboid (both light blue and orange-tan). Also note the variably tarnished overgrowths on euhedral pyrite crystals with no obvious framboidal cores. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 26.28 m (86.22 ft). (B) Photomicrograph of pyrite cement (PCe) that occupies areas between framboids in rimmed framboid concretion. Clearly the cement postdates formation of the framboids within the concretion. Dark areas between the framboids and pyrite cement are Fe-dolomite cement. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 10.32 m (33.86 ft). (C) Photomicrograph of an anhedral pyrite overgrowth (OV) on the surface of a euhedral matrix pyrite crystal (P). The variation in tarnish color between the euhedral crystal (bluish-tan) and the anhedral overgrowth (orange-tan) assists in deciphering the two generations. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 3.70 m (12.14 ft). (D) Photomicrograph of multiple generations of overgrowths on small euhedral pyrite crystals in the matrix. The different generations of overgrowths are apparent based on the variability in tarnish color displayed. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 27.51 m (90.26 ft).

Massive, Anhedral Pyrite

Massive, anhedral pyrite is present in at least two distinct textural forms. First, anhedral pyrite is an accessory mineral in bedding parallel/subparallel fractures, but it appears to be coeval with other phases in the fractures including Fe-dolomite (Figure 6A), bitumen (Figure 6B), barite, chalcedony, and muscovite (mica identification based on XRD analyses). The fractures have apertures up to 0.2 mm and can be relatively continuous horizontally or as en echelon features. Locally, these fractures also exist adjacent to rimmed framboid concretions. Bedding parallel fractures are relatively common throughout the Alum Shale samples studied although pyrite mineralization in them is not ubiquitous (Table 1).

Photographs showing authigenic pyrite that postdates other, previously shown pyrite textures, Alum Shale. (A) Anhedral pyrite (P) that is coeval with Fe-dolomite (D) within “beef.” Photomicrograph taken in vertically reflected light in air; sample is from a depth of 18.30 m (60.04 ft). (B) Photograph of pyrite (P) coeval with bitumen (B) and barite (Ba) in a segment of “beef.” Note how the bitumen conforms to the surface of the pyrite and the barite also conforms to the surface features of the bitumen. Photograph taken in SEM-backscatter mode; sample is from a depth of 7.00 m (22.97 ft). (C) Photograph of hand sample showing the distortion of “beef” by massive pyrite. Also note the presence of a vertical calcite-filled fracture that cuts massive pyrite. Photograph taken of sample from a depth of 12.05 m (39.53 ft).

Photographs showing authigenic pyrite that postdates other, previously shown pyrite textures, Alum Shale. (A) Anhedral pyrite (P) that is coeval with Fe-dolomite (D) within “beef.” Photomicrograph taken in vertically reflected light in air; sample is from a depth of 18.30 m (60.04 ft). (B) Photograph of pyrite (P) coeval with bitumen (B) and barite (Ba) in a segment of “beef.” Note how the bitumen conforms to the surface of the pyrite and the barite also conforms to the surface features of the bitumen. Photograph taken in SEM-backscatter mode; sample is from a depth of 7.00 m (22.97 ft). (C) Photograph of hand sample showing the distortion of “beef” by massive pyrite. Also note the presence of a vertical calcite-filled fracture that cuts massive pyrite. Photograph taken of sample from a depth of 12.05 m (39.53 ft).

A second textural feature of massive, anhedral pyrite is as vertical/subvertical masses that significantly disrupt and distort bedding as well as the bedding parallel fractures (Figure 6C). In hand specimen, these masses can be up to 2 cm (0.8 in.) in vertical extent and up to 1 cm (0.4 in.) in width (Figure 6C). Pyrite is the dominant mineral present in these masses although minor barite, sphalerite, and galena have also been observed intergrown with the pyrite. Massive pyrite in the vertical/subvertical features was observed in only a few Alum samples (Table 1).

Carbonate Mineralization

Fe-Dolomite

Fe-dolomite occurs in several different forms in the Alum Shale. It is the principal mineral in the rims of rimmed framboid concretions. Here, the Fe-dolomite is fibrous and forms an equidimensional rim up to 100 μm in thickness (Figure 7A). The Fe-dolomite also occurs cementing the framboids within rimmed framboid concretions (Figure 7B). Multi-sized, optically continuous crystals of Fe-dolomite cement multiple, adjacent framboids; as such, the Fe-dolomite imparts a micropoikilotopic fabric within these concretions. Fe-dolomite is also present as the dominant mineral in bedding parallel/subparallel fractures (Figure 6A). Here, the Fe-dolomite is fibrous, largely perpendicular to bedding (Figure 7C).

Photographs of authigenic carbonate in the Alum Shale. (A) Photomicrograph of Fe-dolomite (D) that forms on a rimmed framboid concretion. Note the orientation of Fe-dolomite is roughly perpendicular to the surface of the concretion. Photomicrograph taken under crossed-polarized light; sample from a depth of 12.50 m (41.01 ft). (B) Photomicrograph of Fe-dolomite that is present as a micropoikilotopic cement of framboids in a rimmed framboid concretion. Notice that relatively large areas containing multiple framboids, which appear as black masses, are cemented with individual Fe-dolomite crystals that are in optical continuity. Photomicrograph taken under crossed-polarized light; sample is from a depth of 16.82 m (55.18 ft). (C) Photomicrograph of bedding parallel/subparallel “beef” that is largely Fe-dolomite. Note the orientation of the Fe-dolomite crystals is perpendicular to the aperture of the “beef” fractures. Photomicrograph taken under crossed-polarized light; sample is from a depth of 18.30 m (60.04 ft). (D) Photomicrograph taken showing the relative timing between “beef” (horizontally oriented features) and a later vertical calcite-filled fracture that cuts the “beef.” Photomicrograph taken under crossed-polarized light; sample is from a depth of 28.86 m (94.69 ft).

Photographs of authigenic carbonate in the Alum Shale. (A) Photomicrograph of Fe-dolomite (D) that forms on a rimmed framboid concretion. Note the orientation of Fe-dolomite is roughly perpendicular to the surface of the concretion. Photomicrograph taken under crossed-polarized light; sample from a depth of 12.50 m (41.01 ft). (B) Photomicrograph of Fe-dolomite that is present as a micropoikilotopic cement of framboids in a rimmed framboid concretion. Notice that relatively large areas containing multiple framboids, which appear as black masses, are cemented with individual Fe-dolomite crystals that are in optical continuity. Photomicrograph taken under crossed-polarized light; sample is from a depth of 16.82 m (55.18 ft). (C) Photomicrograph of bedding parallel/subparallel “beef” that is largely Fe-dolomite. Note the orientation of the Fe-dolomite crystals is perpendicular to the aperture of the “beef” fractures. Photomicrograph taken under crossed-polarized light; sample is from a depth of 18.30 m (60.04 ft). (D) Photomicrograph taken showing the relative timing between “beef” (horizontally oriented features) and a later vertical calcite-filled fracture that cuts the “beef.” Photomicrograph taken under crossed-polarized light; sample is from a depth of 28.86 m (94.69 ft).

Organic Material

Organic material is present throughout the Alum Shale and is dispersed as a rock constituent throughout all facies. It typically fills areas between detrital quartz, feldspar, and clay minerals (Figure 4A). The organic material has no obvious megascopic shape, internal fabric or structure, and as such is considered amorphous. It also occurs as a partial to complete fill in some bedding parallel fractures (Figure 6B). Present day TOC values range from 7.9 to ≤14 wt. % (Table 2). Rock-Eval pyrolysis data reveal that the HI and OI values are both low (Table 2). Tmax values were deemed unreliable because S2 values are low; low S2 values result in broad peaks on pyrograms, which limit accurate determination of Tmax values (see Peters and Cassa, 1994).

Paragenetic Sequence of Key Alterations

Determination of the relative timing of formation of the minerals and fractures described above was a focus of this work. The paragenetic sequence shown in Figure 8 reflects a composite of relationships throughout the studied interval, and as such, is not to suggest that each feature is present in all samples. Indeed, some features are stratigraphically restricted (Table 1). Thus, the criteria used to develop the relative timing of key events shown in Figure 8 include superpositioning of features, cross-cutting relationships, and geological reasoning.

Schematic diagram showing the relative timing of different diagenetic features observed in the Alum Shale. The onset of Alum deposition in Scania is estimated to be at ~505 Ma based on the trilobite zonation of Ahlberg et al. (2009), and the onset of bitumen generation is extrapolated from work by Pedersen et al. (2007). Syn = syndepositional; HC = hydrocarbon generation; diag = diagenesis.

Schematic diagram showing the relative timing of different diagenetic features observed in the Alum Shale. The onset of Alum deposition in Scania is estimated to be at ~505 Ma based on the trilobite zonation of Ahlberg et al. (2009), and the onset of bitumen generation is extrapolated from work by Pedersen et al. (2007). Syn = syndepositional; HC = hydrocarbon generation; diag = diagenesis.

Possible Syndepositional Pyritization

Most pyrite textures previously described demonstrate features that aid in determining their relative timing of precipitation, although pyrite crystallites have uncertain temporal relationships to other key alterations. The occurrence of crystallites between clay platelets (Figure 4A) points to their formation prior to significant compaction of the platelets against each other. Nevertheless, in the absence of any clear temporal relationship with other diagenetic features, other than compaction, it is inferred that individual pyrite crystallites precipitated syndepositionally or soon after deposition.

Early Authigenesis

Individual framboids in the matrix and in framboid concretions are considered to be early authigenic pyrite based on textural considerations. Clay platelets bend around matrix framboids, which points to their formation prior to significant compaction. It is presumed that matrix framboids formed when there was ample pore space in the sediments to allow grow unimpeded when porosity was abundant in the Alum Shale.

All types of pyrite concretions distort bedding above and below them (Figure 4B, C), indicating that they formed when the sediment was sufficiently plastic to deform around them. Rimless framboid concretions are presumed to have preceded the formation of the rimmed framboid concretions, largely because in the rare instances when both are found in close proximity, rimmed framboid concretions appear to distort rimless framboid concretions. This relationship points to the existence of the rimless concretions in the Alum Shale prior to those that are rimmed. Concretions of euhedral pyrite crystals do not demonstrate any paragenetic relationship with other concretions, but the presence of framboidal textures in the core of some euhedral pyrite (Figures 4F, 5D) indicates that euhedral crystals formed subsequent to framboids.

Cementation by Fe-dolomite of rimmed framboid concretions (Figure 7A) corroborates the early timing of formation of these concretions. Precipitation of largely equidimensional Fe-dolomite rims on these concretions points to an abundance of pore space in the sediment during concretion formation, which corroborates their early diagenetic formation. The rimmed framboid concretions appear to have grown in a center-to-edge textural manner (after Mozley, 1996), in which the final mineral formed in and around them is Fe-dolomite (Figures 4C, 7A). Furthermore, the presence of pore space between the framboids within these concretions indicates that abundant porosity remained after framboid precipitation to allow for cementation by both pyrite (as overgrowths on framboids) and ultimately by Fe-dolomite (Figure 7B). Indeed, the envelopment of multiple adjacent framboids by individual micropoikilotopic Fe-dolomite crystals provides direct indication of the existence of abundant porosity in the concretions that allowed for precipitation of the Fe-dolomite cement (Figure 7B).

Overgrowths of pyrite precipitated on existing forms of pyrite. By partially or completely encasing matrix framboids, framboids in either type of concretion, and euhedral pyrite crystals in the matrix, the paragenetic relationships are clear. Using the differential tarnish observed in petrographic investigation, it is likely that the overgrowths did not form in a single event but instead formed episodically, leaving multiple thin layers of pyrite as overgrowths.

Pyrite Associated with Hydrocarbon Generation

Pyrite associated with bedding parallel/subparallel fractures (Figure 6A) is considered a temporally later generation of pyrite than any previously discussed. The bedding parallel/subparallel fractures locally divert around rimmed framboid concretions, which indicates that the fractures postdate the concretion. Furthermore, the intimate association of organic material (interpreted as originally being bitumen) in the pyrite-bearing fractures indicates that the pyrite accompanied the emplacement of bitumen; it is inferred that the bitumen was internally generated in the Alum at the onset of hydrocarbon generation (Figure 8).

Late Diagenesis

Cross-cutting relationships denote a texturally late episode of pyrite. The massive, anhedral pyrite that occurs in vertical fractures both cuts and distorts bedding parallel/subparallel fractures (Figure 6C), which clearly points to them postdating bedding parallel/subparallel fractures (Figure 8). Although vertical fracture and contained pyrite were found in only a few samples within the studied interval of the Alum Shale (Table 1), they occur in stratigraphically disparate sections of the Alum exposed in the Andrarum-3 core, irrespective of facies (Table 1), and as such seems to be an overprint of late diagenetic pyrite into the Alum.

DISCUSSION

Much of the pyrite in the Alum Shale in the Andrarum-3 core is unequivocally diagenetic in origin, based on the petrographic and textural observations presented herein. In addition to the pyrite, precipitation of other authigenic minerals, mechanical compaction, and at least two different types of fractures of greatly different nature and orientation have been documented in the Alum. As such, the diagenetic history of the formation is complex and records not only changes in pore-water chemistry over time but also physical changes to these rocks.

Processes of Pyritization in the Alum

Based on the work of Wilkins et al. (1996), in which they studied modern sediments, relatively large (≥10 μm in diameter) and variably sized framboidal pyrite are those that formed from diagenetic processes within the sediment. In contrast, smaller (average: ~5 μm across), less variably sized framboids are more characteristic of those that can form in the water column under euxinic conditions and sink to the seafloor because their density prevents further suspension in the water column (Wilkins et al., 1996). Although matrix framboids in the Alum Shale, at least in some parts of the stratigraphic section, are small (≤6 μm across), they are unlikely to have formed from euxinic bottom waters as the rocks in which the small framboids occur also contain ichnological features such as Planolites burrows (Figure 3; Egenhoff et al., 2015) and fecal strings. Thus, at the time of deposition, bottom waters, as well as water in Alum sediments immediately below the sediment–water interface, must have contained at least a limited supply of oxygen to support the organisms that burrowed and produced fecal strings. It then follows that the waters in the sediments were dysoxic, and the overlying water column was likely oxic to dysoxic. Furthermore, the variable size and relatively large (up to 35 μm) diameter of the matrix framboids are textural features that point to a digenetic origin.

Bacterially mediated processes are considered likely mechanisms by which both sulfur and iron were reduced in the Alum Shale as a necessary step in the process of pyritization. Formation of framboidal pyrite in the Alum is considered herein to have resulted from reduction of sulfate-sulfur in the waters in Alum sediments to H2S by sulfate-reducing bacteria (cf. Berner, 1970, 1985; Sweeney and Kaplan, 1973), working under the assumption that these bacteria or similar ones were active in the Cambrian (cf. Seal, 2006). As some of the sulfate-reducing bacteria can be obligate anaerobes and therefore exist in the presence of limited free oxygen (e.g., Madigan et al., 2012), then it is possible that sulfate reduction, along with framboidal pyrite formation, began before fully anoxic conditions prevailed in the Alum sediments.

Reduction of ferric iron (Fe3+), which was probably originally deposited in the Alum Shale as iron oxide coatings on detritus (e.g., clays), to ferrous iron (Fe2+) may have been by bacterial processes as well (cf. Canfield, 1989), although an abiological origin cannot be ruled out (cf. Canfield, 1989; Coleman et al., 1993). Once H2S and Fe2+ were available in the Alum sediments, framboidal pyrite formation could begin, presumably through the multi-step process wherein a metastable iron monosulfide (e.g., greigite or mackinawite) initially formed, followed by reaction with additional dissolved H2S to form stable pyrite crystallites in the framboids (cf. Sweeney and Kaplan, 1973). As framboids in the matrix occurs throughout the Alum in this study, this process is envisioned to have proceeded, irrespective of original depositional facies.

Within the intervals in the Alum Shale that contain framboid concretions, Fe2+, in the presence of H2S favored precipitation of pyrite over other authigenic iron-bearing minerals, including Fe-carbonates (cf. Raiswell et al., 2011). Framboidal pyrite, and other generations of pyrite, would be expected to precipitate as long as Fe2+ and H2S were available and had reached their respective solubility in the pore fluids. The presence of small overgrowths of pyrite on the framboids indicates that pyrite precipitation continued, at least temporarily in the concretions after the framboids were formed. Nevertheless, once one or both constituents reached low concentrations, pyrite formation ceased, as pyrite is the expected sink for both Fe2+ and H2S (cf. Macquaker et al., 2014). In the case of the rimmed framboid concretions, the complete encasement of the framboid concretion by Fe-dolomite rims, as well as cementation of framboids by Fe-dolomite within the concretions (Figure 7B), indicates that during growth, Fe-dolomite was the final mineral to form in these concretions. As such, it is assumed that dissolved sulfide was completely consumed from the pore waters, at least in and near the concretions, leaving available Fe2+ to combine with dissolved carbonate to form the Fe-dolomite. That Fe-dolomite formed in the Alum rimmed framboid concretions rather than ankerite or siderite is likely because Fe2+ concentrations had also fallen relative to dissolved Ca2+, which would then favor Fe-dolomite over ankerite or siderite (cf. Raiswell et al., 2011). Thus, it is envisioned that the rimmed framboid concretions grew in a closed system that ultimately led to localized depletion of dissolved sulfide, which then facilitated precipitation of Fe-dolomite rims.

The presence of Fe-dolomite on and within rimmed framboid concretions, and its absence in rimless concretions, provides the foundation for explaining the divergence of processes that lead to formation of each. For both types of concretions, the accumulation of pyrite framboids in concretionary masses required a fairly continuous supply of H2S and Fe2+ to be fed to a limited spatial area for ongoing formation of framboids. Thus, ample pore space and ready fluid flow would be required for formation of framboid concretions, which is consistent with their early timing of formation (Figure 4B, C). Although both types of concretions required H2S and Fe2+ for framboid formation, it is likely that framboidal pyrite precipitation was more prolonged in rimmed concretions because they tend to be larger and contain more framboids than rimless concretions (Table 1), which points to a more extensive accumulation of framboidal pyrite formation in them. The abundance of framboids in rimmed concretions indicates that bacterial sulfate reduction was quite active, at least until sulfate became limited because of virtually complete bacterial consumption. It is then not surprising that Fe-dolomite formed as the final mineral in these concretions, as sulfur, not iron was the limiting agent in the Alum, much as it was in other systems where dolomite forms in the absence of available sulfur (e.g., Baker and Kastner, 1981; Baker and Burns, 1985; Raiswell et al., 2011; Howe et al., 2016). Thus, the Fe-dolomite rims mark a time in the Alum sediments when there was at least temporary exhaustion of H2S.

Although the formation of rimmed framboid concretions in the Alum Shale might indicate that H2S was a limiting species, the precipitation of subsequent generations of pyrite (see Figure 8) in all intervals of the Alum clearly indicates that pyrite formation resumed after these concretions formed. Thus, additional influx of sulfur, probably reduced in the organic-rich sediments from sulfate that diffused into the Alum sediments from the overlying water column, led to the formation of subsequent generations of pyrite (Figure 4C, F). The textural features and paragenetic relationships outlined herein are consistent with other studies, in which framboids form during early diagenesis, followed by later pyrite overgrowths and euhedral pyrite (cf. Raiswell, 1982; Schieber, 2011). The formation of euhedral pyrite subsequent to framboids is likely because of the slowing of sulfate reduction because sulfate is being delivered to the site of pyritization through diffusion (cf. Raiswell, 1982).

The source of the sulfide in pyrite associated with bedding parallel/subparallel fractures in the Alum Shale is uncertain, but the timing of its formation can be reasonably well constrained. The presence of bitumen in at least some of these fractures (Figure 6B) indicates that they were open features at the time the bitumen was a mobile phase. As bitumen generation is the first step in thermogenic hydrocarbon generation (cf. Peters and Cassas, 1994), it then follows that in the Alum, these fractures and the fill in them are temporally related to the onset of hydrocarbon generation (Fishman et al., 2017). The formation of these bedding parallel/subparallel fractures, considered herein as “beef” (cf. Cobbold and Rodriguez, 2007), probably resulted from overpressuring that commenced in the Alum from hydrocarbon generation (cf. Cobbold and Rodriguez, 2007; Al Duhailan et al., 2015). Based on thermal history reconstruction in southern Scandinavia and extrapolated to the Alum in the study area, the formation appears to have entered the hydrocarbon generation window at about 410 Ma (cf. Pedersen et al., 2007). A source of pyrite that is coeval with “beef” would be unlikely to be related with connate waters in the Alum given the duration between deposition and hydrocarbon generation and the diagenesis that occurred. Hence, an external source of solutes for pyrite to form is likely.

Textural relationships between “beef” and massive pyrite (Figure 6C) indicate that yet another generation of pyrite can be documented in the Alum Shale subsequent to the onset of hydrocarbon generation. The massive pyrite is intergrown with sphalerite, galena, and some barite. Although there are limited data available regarding the hydrothermal history within the area of the Andrarum-3 core locality, several probable hydrothermal veins with galena and fluorite have been observed approximately 10–15 km (6–9 mi) to the southeast (Johansson et al., 1984; Álvaro et al., 2016). These veins may have formed during late Carboniferous to early Permian (Johansson et al., 1984), perhaps from rifting caused by the Variscan Orogeny. Because the massive pyrite formed relatively late in the diagenetic history of the Alum, and because galena is intergrown with the massive pyrite, it is possible that the massive pyrite in the Alum was genetically related to the late Carboniferous to early Permian veins (see Figure 8).

Pyrite Textures and the SPICE Interval

The textural and paragenetic relationships of different generations of pyrite in the Alum Shale indicate that pyritization was a result of diagenetic processes, including in the peak SPICE interval, where a sulfur isotope excursion has been documented on bulk samples (Gill et al., 2011). A diagenetic origin for the pyrite in the Alum is the logical conclusion drawn from the petrographic data, but this mechanism of pyrite formation is markedly different from that concluded by Gill et al. (2011). Thus, it seems appropriate to consider the textural data within the context of the bulk sample sulfur isotopic data of Gill et al. (2011) in an attempt to consider other explanations for the isotopic data.

Of all the texturally and temporally different generations of pyrite in the Alum Shale, two textures—rimless and rimmed framboid concretions—appear to be coincident with the interval that was outlined in Gill et al. (2011) to be the peak of the sulfur isotope excursion that they used to outline the presumed SPICE event (Table 1) in the Alum. Furthermore, the limited XRD data generated in this study show generally higher amounts of pyrite within the SPICE interval than above or below it (Table 1). The somewhat restricted interval in which these concretions occur indicates that those processes responsible for their formation were limited to these portions of the Alum. Other textures of pyrite are either pervasive throughout the Alum in the Andrarum-3 core (Table 1) or somewhat random and seemingly spatially unrelated to the presumed SPICE isotopic excursion.

In the absence of detailed sulfur isotopic data as a function of different pyrite generations, it is not possible to definitively link pyrite textures to sulfur isotopic trends in the Alum Shale. Nevertheless, the petrographic evidence presented herein provides clues as to the processes acting during various times, which can then be useful in postulating a possible cause of the sulfur isotopic variation documented by Gill et al. (2011). Rimmed framboidal pyrite concretions seem to have formed in a closed sulfide system, which is why Fe-dolomite is the last mineral to form in them. With closed-system behavior, an isotopic excursion toward heavier sulfur isotopic values could have been expected for the framboids in these concretions (cf. Seal, 2006). Although a closed system is considered in the formation of these concretions, the presence of subsequent generations of pyrite indicates that after the concretions formed, an influx of sulfide occurred that led to precipitation of the succeeding generations pyrite outlined in Figure 8.

Although the rimless framboid concretions do not contain Fe-dolomite cement, the accumulation of many framboids in them may have led to a significant drop in the availability of dissolved sulfide during the time when these concretions formed. It is unclear if the rimless framboid concretions formed fully in a closed sulfide system, but precipitation of the numerous framboids in them may have significantly depleted the available dissolved sulfide. Given that both types of concretions are largely restricted to the zone of isotopic excursion noted by Gill et al. (2011), it is plausible that their presence in this stratigraphically restricted interval may be responsible for the bulk isotopic shift identified by Gill et al. (2011).

CONCLUSIONS

Careful petrographic analysis of the Alum Shale provides important information that assists in unraveling the complex alteration history experienced by it. The Alum, as revealed from this study, experienced precipitation of multiple generations of authigenic pyrite, some of which formed many millions of years after the Alum was deposited. Thus, chemically reducing conditions prevailed during diagenesis in the Alum, which resulted in periodic precipitation of texturally distinct pyrite generations. That some of the pyrite is coeval with the onset of hydrocarbon generation, which occurred many millions of years after deposition, indicates that pyritization in the Alum was at least a recurrent diagenetic feature throughout the postdepositional history of the formation.

The restriction of two types of framboidal pyrite concretions largely within the SPICE interval of the Alum Shale indicates that processes within this interval differed, at least at times, to those elsewhere in the Alum. Framboid concretions that are rimmed by Fe-dolomite, and within which the framboids are cemented by Fe-dolomite, indicate that the iron supply was not limited in the Alum throughout its diagenetic evolution.

ACKNOWLEDGMENTS

This chapter was greatly improved by the technical reviews of Peir Pufhal, an anonymous reviewer, and Alan Koenig, and we are grateful to them for their comments and suggestions. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Measured section of the Andrarum-3 drill core (modified from Egenhoff et al., 2015) showing the stratigraphic position, within the Alum Shale, of samples used in this study (sample designations are all AN-10- with the addition of the sample depth, in meters). The SPICE interval, as shown, and the sulfur isotopic data are from Gill et al. (2011). The trilobite zones are from Ahlberg et al. (2009).

Measured section of the Andrarum-3 drill core (modified from Egenhoff et al., 2015) showing the stratigraphic position, within the Alum Shale, of samples used in this study (sample designations are all AN-10- with the addition of the sample depth, in meters). The SPICE interval, as shown, and the sulfur isotopic data are from Gill et al. (2011). The trilobite zones are from Ahlberg et al. (2009).

Photomicrograph of a Planolites burrow (encircled by white dashed line) in the Alum Shale. Note that inside the burrow, there are largely detrital mineral grains (largely clay minerals with some silt-zie quartz and other detritus) and a paucity of organic material and pyrite whereas outside of the burrow organic material and pyrite are abundant and appear as black material in matrix. The burrow is parallel to bedding. Photomicrograph taken in plane, transmitted light, and the sample is from a depth of 12.95 m (42.49 ft).

Photomicrograph of a Planolites burrow (encircled by white dashed line) in the Alum Shale. Note that inside the burrow, there are largely detrital mineral grains (largely clay minerals with some silt-zie quartz and other detritus) and a paucity of organic material and pyrite whereas outside of the burrow organic material and pyrite are abundant and appear as black material in matrix. The burrow is parallel to bedding. Photomicrograph taken in plane, transmitted light, and the sample is from a depth of 12.95 m (42.49 ft).

Photographs of authigenic sulfide textures from the samples of the Alum Shale. (A) SEM image of pyrite crystallites (arrows) within organic material (dark η) that surrounds detrital clay minerals. Note that the individual pyrite crystallites are ≪1 μm across. Also note the present of framboids (B) of varying sizes. Photograph taken in SEM-backscatter mode from sample depth of 7.00 m (22.97 ft). (B) Photomicrograph of a rimless framboid concretion (arrows) that is elongate parallel (horizontal) to bedding. Note that concretion is composed of many individual framboids. Photomicrograph taken in vertically reflected light in air; sample is from depth of 12.00 m (39.37 ft). (C) Photograph of a rimmed framboid concretion (rim outlined in white, dashed line). The interior of the concretion is composed of numerous individual framboids, whereas the rim is largely Fe-dolomite (determined from SEM–EDS analyses). Note that bedding is distorted above and below concretion. Photograph taken in SEM-backscatter mode from a sample depth of 12.50 m (41.01 ft). (D) Photomicrograph of euhedral pyrite grain (center of photo), with numerous smaller subhedral pyrite crystals (white arrows) and anhedral pyrite crystals (yellow arrows) in the surrounding matrix. Note how bedding is distorted around the large euhedral pyrite crystal. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 12.50 m (41.01 ft). (E) Photomicrograph of a concretion composed of euhedral pyrite crystals. Note that the pyrite demonstrates various colors resulting from tarnish; it remains unclear why the crystals display a differing tarnish. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 2.77 m (9.09 ft). (F) Photomicrograph of euhedral pyrite crystals dispersed in the matrix of the Alum Shale. Note the presence of a framboid at the core of many of the pyrite crystals (arrows) that now display a euhedral texture. The tarnish of the rims of authigenic pyrite on framboids assists in identification of the framboids within the euhedral pyrite. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 21.00 m (68.90 ft).

Photographs of authigenic sulfide textures from the samples of the Alum Shale. (A) SEM image of pyrite crystallites (arrows) within organic material (dark η) that surrounds detrital clay minerals. Note that the individual pyrite crystallites are ≪1 μm across. Also note the present of framboids (B) of varying sizes. Photograph taken in SEM-backscatter mode from sample depth of 7.00 m (22.97 ft). (B) Photomicrograph of a rimless framboid concretion (arrows) that is elongate parallel (horizontal) to bedding. Note that concretion is composed of many individual framboids. Photomicrograph taken in vertically reflected light in air; sample is from depth of 12.00 m (39.37 ft). (C) Photograph of a rimmed framboid concretion (rim outlined in white, dashed line). The interior of the concretion is composed of numerous individual framboids, whereas the rim is largely Fe-dolomite (determined from SEM–EDS analyses). Note that bedding is distorted above and below concretion. Photograph taken in SEM-backscatter mode from a sample depth of 12.50 m (41.01 ft). (D) Photomicrograph of euhedral pyrite grain (center of photo), with numerous smaller subhedral pyrite crystals (white arrows) and anhedral pyrite crystals (yellow arrows) in the surrounding matrix. Note how bedding is distorted around the large euhedral pyrite crystal. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 12.50 m (41.01 ft). (E) Photomicrograph of a concretion composed of euhedral pyrite crystals. Note that the pyrite demonstrates various colors resulting from tarnish; it remains unclear why the crystals display a differing tarnish. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 2.77 m (9.09 ft). (F) Photomicrograph of euhedral pyrite crystals dispersed in the matrix of the Alum Shale. Note the presence of a framboid at the core of many of the pyrite crystals (arrows) that now display a euhedral texture. The tarnish of the rims of authigenic pyrite on framboids assists in identification of the framboids within the euhedral pyrite. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 21.00 m (68.90 ft).

Photomicrographs of different generations of authigenic pyrite in the Alum Shale. (A) Photomicrograph of overgrowths (OV) of pyrite on matrix framboids (B) as well as euhedral pyrite crystals. The variability of the tarnish of the pyrite assists in observing the various generations of pyrite observed associated with the framboids, including the pyrite crystals that comprise the framboid (light blue), pyrite that cements the intraframboidal region (orange-tan), and overgrowths on the entire framboid (both light blue and orange-tan). Also note the variably tarnished overgrowths on euhedral pyrite crystals with no obvious framboidal cores. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 26.28 m (86.22 ft). (B) Photomicrograph of pyrite cement (PCe) that occupies areas between framboids in rimmed framboid concretion. Clearly the cement postdates formation of the framboids within the concretion. Dark areas between the framboids and pyrite cement are Fe-dolomite cement. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 10.32 m (33.86 ft). (C) Photomicrograph of an anhedral pyrite overgrowth (OV) on the surface of a euhedral matrix pyrite crystal (P). The variation in tarnish color between the euhedral crystal (bluish-tan) and the anhedral overgrowth (orange-tan) assists in deciphering the two generations. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 3.70 m (12.14 ft). (D) Photomicrograph of multiple generations of overgrowths on small euhedral pyrite crystals in the matrix. The different generations of overgrowths are apparent based on the variability in tarnish color displayed. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 27.51 m (90.26 ft).

Photomicrographs of different generations of authigenic pyrite in the Alum Shale. (A) Photomicrograph of overgrowths (OV) of pyrite on matrix framboids (B) as well as euhedral pyrite crystals. The variability of the tarnish of the pyrite assists in observing the various generations of pyrite observed associated with the framboids, including the pyrite crystals that comprise the framboid (light blue), pyrite that cements the intraframboidal region (orange-tan), and overgrowths on the entire framboid (both light blue and orange-tan). Also note the variably tarnished overgrowths on euhedral pyrite crystals with no obvious framboidal cores. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 26.28 m (86.22 ft). (B) Photomicrograph of pyrite cement (PCe) that occupies areas between framboids in rimmed framboid concretion. Clearly the cement postdates formation of the framboids within the concretion. Dark areas between the framboids and pyrite cement are Fe-dolomite cement. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 10.32 m (33.86 ft). (C) Photomicrograph of an anhedral pyrite overgrowth (OV) on the surface of a euhedral matrix pyrite crystal (P). The variation in tarnish color between the euhedral crystal (bluish-tan) and the anhedral overgrowth (orange-tan) assists in deciphering the two generations. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 3.70 m (12.14 ft). (D) Photomicrograph of multiple generations of overgrowths on small euhedral pyrite crystals in the matrix. The different generations of overgrowths are apparent based on the variability in tarnish color displayed. Photomicrograph taken in vertically reflected light in air; sample is from a depth of 27.51 m (90.26 ft).

Photographs showing authigenic pyrite that postdates other, previously shown pyrite textures, Alum Shale. (A) Anhedral pyrite (P) that is coeval with Fe-dolomite (D) within “beef.” Photomicrograph taken in vertically reflected light in air; sample is from a depth of 18.30 m (60.04 ft). (B) Photograph of pyrite (P) coeval with bitumen (B) and barite (Ba) in a segment of “beef.” Note how the bitumen conforms to the surface of the pyrite and the barite also conforms to the surface features of the bitumen. Photograph taken in SEM-backscatter mode; sample is from a depth of 7.00 m (22.97 ft). (C) Photograph of hand sample showing the distortion of “beef” by massive pyrite. Also note the presence of a vertical calcite-filled fracture that cuts massive pyrite. Photograph taken of sample from a depth of 12.05 m (39.53 ft).

Photographs showing authigenic pyrite that postdates other, previously shown pyrite textures, Alum Shale. (A) Anhedral pyrite (P) that is coeval with Fe-dolomite (D) within “beef.” Photomicrograph taken in vertically reflected light in air; sample is from a depth of 18.30 m (60.04 ft). (B) Photograph of pyrite (P) coeval with bitumen (B) and barite (Ba) in a segment of “beef.” Note how the bitumen conforms to the surface of the pyrite and the barite also conforms to the surface features of the bitumen. Photograph taken in SEM-backscatter mode; sample is from a depth of 7.00 m (22.97 ft). (C) Photograph of hand sample showing the distortion of “beef” by massive pyrite. Also note the presence of a vertical calcite-filled fracture that cuts massive pyrite. Photograph taken of sample from a depth of 12.05 m (39.53 ft).

Photographs of authigenic carbonate in the Alum Shale. (A) Photomicrograph of Fe-dolomite (D) that forms on a rimmed framboid concretion. Note the orientation of Fe-dolomite is roughly perpendicular to the surface of the concretion. Photomicrograph taken under crossed-polarized light; sample from a depth of 12.50 m (41.01 ft). (B) Photomicrograph of Fe-dolomite that is present as a micropoikilotopic cement of framboids in a rimmed framboid concretion. Notice that relatively large areas containing multiple framboids, which appear as black masses, are cemented with individual Fe-dolomite crystals that are in optical continuity. Photomicrograph taken under crossed-polarized light; sample is from a depth of 16.82 m (55.18 ft). (C) Photomicrograph of bedding parallel/subparallel “beef” that is largely Fe-dolomite. Note the orientation of the Fe-dolomite crystals is perpendicular to the aperture of the “beef” fractures. Photomicrograph taken under crossed-polarized light; sample is from a depth of 18.30 m (60.04 ft). (D) Photomicrograph taken showing the relative timing between “beef” (horizontally oriented features) and a later vertical calcite-filled fracture that cuts the “beef.” Photomicrograph taken under crossed-polarized light; sample is from a depth of 28.86 m (94.69 ft).

Photographs of authigenic carbonate in the Alum Shale. (A) Photomicrograph of Fe-dolomite (D) that forms on a rimmed framboid concretion. Note the orientation of Fe-dolomite is roughly perpendicular to the surface of the concretion. Photomicrograph taken under crossed-polarized light; sample from a depth of 12.50 m (41.01 ft). (B) Photomicrograph of Fe-dolomite that is present as a micropoikilotopic cement of framboids in a rimmed framboid concretion. Notice that relatively large areas containing multiple framboids, which appear as black masses, are cemented with individual Fe-dolomite crystals that are in optical continuity. Photomicrograph taken under crossed-polarized light; sample is from a depth of 16.82 m (55.18 ft). (C) Photomicrograph of bedding parallel/subparallel “beef” that is largely Fe-dolomite. Note the orientation of the Fe-dolomite crystals is perpendicular to the aperture of the “beef” fractures. Photomicrograph taken under crossed-polarized light; sample is from a depth of 18.30 m (60.04 ft). (D) Photomicrograph taken showing the relative timing between “beef” (horizontally oriented features) and a later vertical calcite-filled fracture that cuts the “beef.” Photomicrograph taken under crossed-polarized light; sample is from a depth of 28.86 m (94.69 ft).

Schematic diagram showing the relative timing of different diagenetic features observed in the Alum Shale. The onset of Alum deposition in Scania is estimated to be at ~505 Ma based on the trilobite zonation of Ahlberg et al. (2009), and the onset of bitumen generation is extrapolated from work by Pedersen et al. (2007). Syn = syndepositional; HC = hydrocarbon generation; diag = diagenesis.

Schematic diagram showing the relative timing of different diagenetic features observed in the Alum Shale. The onset of Alum deposition in Scania is estimated to be at ~505 Ma based on the trilobite zonation of Ahlberg et al. (2009), and the onset of bitumen generation is extrapolated from work by Pedersen et al. (2007). Syn = syndepositional; HC = hydrocarbon generation; diag = diagenesis.

Table 1.

Petrologic data on different pyrite textures, Alum Shale samples. *, range based on the diameters of at least 10 randomly selected samples; nd, No data; No, not observed; Present, feature observed but not quantified or measured.

Formation of sygenetic and early diagenetic iron minerals in the late Archean Mt. McRae Shale, Hamersley Basin, Australia—New insights on the patterns, controls, and environmental implications of authigenic mineral formation